Conventionally, a propylene-based resin composition has excellent characteristics such as rigidity, heat resistance and chemical resistance, and a piping member made of a propylene-based resin composition is widely used in various types of industrial installations and in the fields of medical care and construction. In particular, a pipe made of a propylene-based resin has chemical resistance to acid and alkali in a high-temperature range (60 to 95° C.) and has a low price, and is therefore suited as an industrial high-temperature piping member through which a chemical solution flows at a high temperature. Regarding such piping members for high temperature, conventional metal pipes tend to be replaced by resin pipes and also a propylene-based resin is candidate for the material of the pipe, and thus increased application is expected in the future. A resinous high-temperature piping member must withstand use over a long period while allowing liquid to flow at a high temperature of 95° C. (for example, when SDR (outer diameter/wall thickness) is 11, the piping member must not fail under a load of 0.4 MPa for 10 years). In order to meet these requirements, excellent high-temperature creep characteristics are required.
Previous proposed propylene-based resin compositions having improved high-temperature creep characteristics include a resin composition which has an ethylene content of 0.1 to 2% by weight, a melt flow index (the same as a melt flow rate) of 5 dg/m or less (corresponding to 5 g/10 min) and a molecular weight distribution of 6 to 20 (refer to Japanese Unexamined Patent Publication (Kokai) No. 10-53628). The high-temperature creep characteristics of this resin are in conformity with DIN 8078 (at 95° C. under a test stress of 3.5 N/mm2) and satisfy a fracturing time of a piping member of 1,200 hours or more (2,000 or more hours in the highest numerical value) and a minimum fracturing time in accordance with DIN 8078 of 1,000 hours or more.
However, when the conventional resin composition is used as a high-temperature piping member through which a chemical solution at a high temperature flows over a long period, higher-temperature creep characteristics are required. Since the molecular weight distribution (hereinafter referred to as Mw/Mn) is within a wide range from 6 to 20 and Mw/Mn is within a wide range, a high level of low molecular weight components is contained. Therefore, in the fields of food, medical care and semiconductor production, in a piping line enclosing a flow of pure water or a chemical solution for which an elution of the piping member is extremely undesirable, there was a problem that a low molecular weight component is eluted into the fluid. Also there was a problem that, when the low molecular weight component is contained, the piping member is likely to deteriorate, resulting in deterioration of chemical resistance. Furthermore, there was a problem that, since low-temperature brittleness cannot be suppressed as a result of deterioration of impact resistance (refer to Comparative Example 8: corresponding to a conventional resin composition), when the piping member is used for outdoor piping in an environment where an outdoor winter temperature is −20° C. or lower, the fluid becomes frozen and expands when flow of the fluid is stopped, and thus the piping member may be fail.
Although there is a method in which an amount of a rubber component in a conventional resin composition increases so as to suppress low-temperature brittleness, since high-temperature creep characteristics deteriorate when the amount of the rubber component increases, it was difficult to simultaneously satisfy physical strength at high temperatures and low temperatures to thus obtain a piping member having a wide application temperature range even when only the amount of the rubber component is increased.
Another method of improving high-temperature creep characteristics includes a method in which 25 to 65% by weight of glass filaments having a length of 2 to 50 mm are mixed with 35 to 75% by weight of a matrix polymer having a melt flow rate of 50 g/10 min or more which is a modified propylene homopolymer or modified propylene-ethylene copolymer prepared by modifying an isotactic propylene homopolymer or a crystalline propylene-ethylene copolymer comprising a homopolymer moiety and an isotactic propylene-ethylene copolymer moiety with an unsaturated carboxylic acid or its anhydride, or a mixture of the modified propylene homopolymer or the modified propylene-ethylene copolymer with the unmodified propylene homopolymer or the unmodified propylene-ethylene polymer to give a long fiber-reinforced polypropylene resin composition, and then the long fiber-reinforced polypropylene resin composition is molded to obtain a molded article (refer to Japanese Unexamined Patent Publication (Kokai) No. 2002-212364).
The conventional resin composition has excellent high-temperature creep characteristics but contains long glass fibers. Therefore, when the resin composition is used as a piping material to be formed into a piping, long glass fibers may fall off into the fluid which flows through the piping. Also, there is a problem that glass may be attacked according to the kind of fluid flowing through the piping. Furthermore, there is a problem that a piping obtained by extrusion molding of the conventional resin composition is not suitable for use as a piping member since inner surface smoothness is deteriorated.